1. Field of the Invention
The present invention relates to a pulse Doppler radar system having a variable pulse repetition frequency, and in particular to such a system having a means for vectorial integration of the reflected pulses.
2. Description of the Prior Art
In order to improve the detection probability of objects in flight, it is known to significantly increase the signal-to-noise ratio by means of integration of all of the reflected pulses received from the same target. Even small moving target signals can be detected by this measure in the superimposed noise. Summation of the successive reflected pulses from a target can be undertaken by a coherent integration before rectification, or by an incoherent integration of the video signals after rectification. Although coherent integration requires a greater circuit outlay, a better signal-to-noise ratio is obtained by this means.
If a pulse Doppler radar employs a fixed pulse repetition frequency, a vectorial addition of the individual reflected pulses in proper phase relation can be executed by means of a so-called comb filter. This repesents an optimum solution for detection of a moving target. In order to cover the entire Doppler frequency range, the number of filters required is equal to the number of integrated reflected pulses because of the comb structure.
In order to eliminate blind speed regions of a pulse Doppler radar system, it is also known to operate with changing pulse repetition frequencies. The use of a number of pulse repetition frequencies, however, results in the loss of the comb structure of the moving target filters. When the pulse Doppler radar operates with a statistical pulse repetition frequency, the entire Doppler frequency range of interest mast be covered by a large number of coherently integrating filters. Even though a quasi-statistical pulse repetition frequency is employed in practice, whereby the pulse periods of different lengths repeat after a concluded integration interval, filters must nonetheless be distributed over the entire range for which unambiguity is desired. The application of coherent integration in pulse Doppler radar with quasi-statistical pulse repetition frequency is not justifiable for economic reasons.